1. Field of the Invention
The present invention relates to a noise estimation scheme in a communication system, and more particularly to a noise estimation apparatus and related method thereof that uses a constellation diagram regarding modulation and related calculations to achieve the objectives of high precision noise estimation.
2. Description of the Prior Art
In the fields of electronics and communications, noise is always inevitable. Since noise is not able to be wholly separated from either an electronic system or a communication system, it is very helpful to a following signal process if current noise in a system can be estimated.
A constellation diagram is a representation of a signal modulated by a digital modulation scheme such as quadrature amplitude modulation (QAM) or phase-shift keying (PSK), both of which are often used in communication systems.
Each symbol in a communication signal is firstly mapped to constellation points on a constellation diagram at the transmitting end. However, an actual position on the constellation of the symbol received at the receiving end may deviate from the ideal position of an original mapped constellation point on the constellation diagram because of the existence of noise.
When a communication signal of a communication system is received at the receiving end, a system that performs a demodulation process will determine which constellation point a symbol in the communication signal is mapped to, thereby obtaining information carried by each symbol. A received symbol, however, may be received at the position on the constellation diagram that deviates from the ideal position of the constellation point which the received symbol should be located at, because of a variety of noise, such as an additive white Gaussian noise, or/and a phase noise. In these circumstances, by measuring deviation between the ideal position of the constellation point on the constellation diagram that the symbol is originally mapped to and the actual position where the symbol is received on the constellation diagram, an Euclidean distance is acquired, which can be utilized for estimating an amount of noise existing in the communication system, enabling users to comprehend the extent of distortion and interference in the signal.
A constellation point on the constellation diagram which is the nearest to the actual position of the received symbol will be selected as the demodulation result regarding that received symbol. In other words, a received symbol is evaluated as a certain symbol corresponding to the constellation point whose position is nearest to the position of the received symbol. However, the constellation point selected in the demodulation process is probably not the same as which the received symbol is originally mapped to. If the influence caused by noise is very serious, the actual position of the received symbol may be too close to other constellation points on the constellation diagram and therefore the received symbol will be evaluated as the symbol that is mapped to another constellation point at the transmitting end rather than the original constellation point.
Detailed descriptions about the inaccuracy of noise estimation caused by the above-mentioned mistaken evaluation for received symbols are explained in the following. Please refer to FIG. 1, which illustrates symbol mapping corresponding to a 16-QAM on a constellation diagram. A certain symbol is mapped to the constellation point 11 on the constellation diagram at the transmitting end. However, that symbol is actually received at the point 12 on the constellation diagram at the receiving end, which is caused by some kind of noise. Thus, the Euclidean distance between the constellation point 11 and the point 12 can theoretically be measured and used to calculate the noise power. However, the actual position of the received symbol is so close to the constellation point 13 that a slicer at the receiving end wrongly evaluates the received symbol as the symbol corresponding to the constellation point 13, which becomes a mistaken evaluation. Noise in the system is underestimated since the mistaken Euclidean distance between the constellation point 13 and the point 12 will be utilized for calculating noise power rather than the correct Euclidean distance between the constellation point 11 and the point 12, which further causes an underestimation of noise power.